Studies of granules of various digestate and wood ash mixtures

Latvian biogas plants use various organic agricultural waste as raw materials, first of all cattle and pig manure. Digestate, a processed product obtained by anaerobic fermentation, is a good fertilizer. The addition of wood ash could further improve its properties, especially on acid soils. In order for it to be advantageous to transport this fertilizer to further fields, it would be desirable to granulate this fertilizer a mixture of ash and digestate. In this study, granules were prepared from ash and digestate, which were predominantly cattle manure mixtures, and also from ash and digestate, which was predominantly pig manure. With wood ash of the heat production company Fortum, the digestate was prepared in four different mixtures and granulated. The granules were dried with a stream of warm air (40-50 oC) and also naturally kept at laboratory temperature for thirty days. The dried granules were subjected to strength tests. Granules produced from pig or cattle manure with ashes at different proportions were tested for breaking durability. The material testing device INSTRON (accuracy class 0.5) was used to determine the compressive strength of the granules placed upright on the end surface. To increase the accuracy of measurements 5 batches each consisting of 10 granules (diam. 6 mm, length 10 mm) were tested, and the average breaking force per single granule was calculated. The highest strength in longitudinal direction 60.5 N was determined for the granules made from the pig farm biogas plant digestate: ash at a ratio of 3:1, and the lowest strength 41.0 N was found for the granules composed of the pig farm biogas plant digestate: ash in a ratio of 4:1 or less by 32% compared to the most durable granules. High average strength 67.2 N was found in the granules made from the cattle farm biogas plant digestate and ash at a ratio of 1:1, but the granules made from the cattle farm biogas plant digestate and ash at a ratio of 3:1 withstand a compression force 49.0 N or less by 27% compared to the most durable granules. However, both artificially and naturally dried granules are still sufficiently stable and suitable for transport and fertilization.